1. Field of the Invention
The present invention relates to an epitaxial wafer and a method of producing the same. Priority is claimed on Japanese Patent Application No. 2006-240962, filed Sep. 6, 2006, and Japanese Patent Application No. 2007-228716, filed on Sep. 4, 2007, the content of which is incorporated herein by reference.
2. Description of Related Art
There is a trend to miniaturize substrates of semiconductor devices produced using silicon wafers, for example, so as to reduce the production cost of semiconductor devices. In accordance with this trend, there is an increasing demand for improving surface roughness and surface flatness of semiconductor wafers. In addition, with regard to light point defects (LPDs) on a semiconductor wafer, reduction of size and numbers of the LPDs are further required.
In many cases, conventionally used silicon substrates are produced using epitaxial wafers obtained from (100) crystal (hereafter, (100) crystal denotes a crystal for obtaining {100} wafers: wafers having a main surface constituted of (100) plane). Recently, substrates produced using epitaxial wafers obtained from (110) crystal (hereafter, (110) crystal denotes a crystal for obtaining {110} wafers: wafers having a main surfaces constituted of (110) plane) are recognized as substrates of next generation which are appropriately applicable for MOS structure devices such as MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), MPUs (Micro-Processing Units), and Logic Arrays (e.g., Patent Reference 1: Japanese Unexamined Patent Application, First Publication No. 2001-253797). Especially for double face polished {100} substrates having a diameter of not less than 8 inches, reduction of the production costs in consistent with improvement of device property is considered.
An epitaxial wafer made of a (100) crystal has an excellent flatness (surface roughness) which may be reflected on detection limit (minimum size to be detected) of LPD size (size of light point defect) of 100 nm or less. On the other hand, an epitaxial wafer made of a (110) crystal has a large surface roughness compared to the epitaxial wafer made of a (100) crystal. Therefore, the detection limit of the LPD size is increased and small sized LPDs cannot be detected. Where numerous LPDs exist on the main surface of the wafer, the LPDs cause a problem of failure leak current in a device production process. It is considered that inferior surface roughness and larger detection limit of LPDs of the epitaxial wafer made of a (110) crystal compared to the epitaxial wafer made of an (100) crystal are caused by difference of physical properties such as atomic arrangement.
As a method for improving the surface roughness of a wafer to a level nearly the same as the substrate, there is a known method, for example as described in Patent Reference 2 (Japanese Examined Patent Application, Second Publication H8-17163), where after epitaxial growth of an epitaxial layer on a wafer, the main surface of the wafer is mirror polished so as to remove defects such as crowns generated by the epitaxial growth, thereby preventing the generation of dusts.
The above-described method is not sufficient for a case where an LPD size of 100 nm or less is required for a product quality. In the above-described method, there is a possibility that a state of LPDs after the polishing and qualities of surface roughness and surface flatness are changed depending on the surface state after the epitaxial growth. Especially, in the case of a (110) wafer, because of its large surface roughness after the epitaxial growth compared to the (100) wafer, surface roughness and flatness cannot be improved sufficiently by mirror polishing after the growth of the epitaxial layer.
When LPDs existing on the main surface of a wafer are measured using a laser light, degree of flatness has a large influence on the detection noise in the measurement of LPDs. Specifically, detection noise in the time of detecting LPDs increases with decreasing quality of surface roughness. Therefore, in the currently used conventional epitaxial wafer made of an (110) crystal, because of inferior surface roughness of the wafer, LPDs smaller than 100 nm in size cannot be detected by the measurement. In such surface conditions, it is difficult to correctly detect LPDs existing on the main surface of next generation type epitaxial wafers made of (110) crystal to which an LPD size of 100 mm or less is required.
Where the polishing after the epitaxial growth is performed to achieve large removal amount, it is not preferable since unevenness in the thickness is increased and flatness of the wafer is deteriorated. On the other band, where the polishing after the epitaxial growth is performed to achieve a small removal amount, the wafer has a large surface roughness, the detection limit of LPD size is larger than 100 nm, and small sized LPDs cannot be detected.
An object of the present invention is to provide, as an intermediate treatment between the epitaxial growth and surface polishing, low-temperature oxide film formation treatment of a main surface of a wafer using a treatment liquid (chemicals, chemical liquid) of a predetermined composition, thereby providing a method of producing an epitaxial wafer, which enables refining of detection limit of LPD size. Another object of the present invention is to provide an epitaxial wafer produced by the above-described method.